1. Field of the Invention
This disclosure relates to systems and methods for determining olefin concentrations in an olefin-containing fuel. More particularly, this disclosure discusses methods for determining olefin concentrations in gasoline mixtures through the use of Raman spectroscopy and a pure reference sample.
2. Background and Related Art
The refining and processing of crude petroleum into commercially useful petroleum products is a vital industry around the world. One of the most important petroleum products is the class of gasoline fuels. Generally, gasoline fuels comprise a mixture of various hydrocarbon compounds, which typically have between 4 and 12 carbon atoms per molecule. Some examples of such hydrocarbons can include n-paraffins, naphthenes, olefins, and a variety of aromatic compounds, such as toluene and benzene. The concentration and chemical grouping of the various hydrocarbons determines the resulting properties of the gasoline fuel, such as the fuel's octane rating.
The octane rating for a gasoline fuel is defined in terms of its pre-detonation, or knocking, characteristics relative to a standard blend of isooctane (2,3,4-trimethylpentane) and n-heptane. Arbitrarily, an octane number of zero has been assigned to n-heptane and a rating of 100 to isooctane. Thus, an unknown fuel having a knocking tendency equal to a blend of 90% isooctane and 10% n-heptane, by volume, is assigned an octane number of 90.
Because a gasoline fuel with a higher octane rating may be sold at an increased price, many gasoline fuel producers seek to increase gasoline octane ratings in a manner that does not significantly increase the production costs of the fuel. Gasoline producers may increase a gasoline fuel's octane rating in a variety of manners. For example, a producer may enhance a fuel's octane rating by adding isooctane, aromatics, and/or olefins to the fuel. However, because isooctane and aromatics tend to be more expensive than olefins, many gasoline producers prefer to increase a fuel's octane rating through the addition of olefins.
Nevertheless, because olefins may be photo-reactive and form smog when burned in internal combustion engines, many governments across the world limit olefin levels in gasoline. In many cases, these olefin regulations continue to become more stringent. Accordingly, to ensure high octane levels and compliance with government environmental regulations, many gasoline producers seek to measure the olefin levels in the gasoline fuels they produce.
Currently, olefin levels in gasoline are measured and predicted in a variety of ways, including through the use of supercritical fluid chromatography (“SFC”, ASTM-D6550-05), fluorescent indicator absorption (“FIA”, ASTM-D1319), and chemometric modeling. However, such techniques may have significant shortcomings. By way of example, SFC may require the use of expensive equipment and chemicals at high pressures, which make the chemicals hard to handle. For instance, SFC may require the use of high purity carbon dioxide (e.g., 99.99% pure), high purity nitrogen (e.g., 99.99% pure), and/or hydrocarbon-free air (e.g., a very clean compressed air) at pressures that are greater than 3,000 psi. Additionally, certain SFC techniques are inaccurate at determining the olefin concentrations from multiple gasoline samples, especially where the olefin levels are relatively high. Because such SFC techniques may have a relatively high standard error, in order to comply with government regulations, many gasoline producers must limit the olefin content in their gasoline to an amount within the error of the detection technique. Such gasoline producers could include more olefins in gasoline if the producers were able to more accurately measure the olefin levels.
In another example, FIA may have several shortcomings. For instance, FIA can be a relatively time consuming process. Indeed, in some cases, an FIA testing procedure may take from about 2 to about 3 hours from start to finish. Additionally, FIA may not be suitable for use with fuels that contain alcohol, such as methanol, ethanol, butanol, and other oxygenates such as tertiary-amyl methyl ether (TAME) and methyl tertiary butyl ether (MTBE), and so forth. The disadvantages associated with this inability to properly test alcohol-containing fuels can become even more pronounced as fuel producers try to conserve oil by blending fuels with alcohol. Furthermore, FIA testing procedures may often be inaccurate from one test to another or from user to another. For instance, FIA requires a user to pack a column with a silica gel that is used to separate the various components of the fuel (e.g., paraffins, aromatics, olefins, etc.). However, because one user may pack one column differently than another column, or because one user may pack a column differently than another user, the FIA results from one column may vary from the results of another column.
In some cases, a gasoline that contains lower olefin levels may have a lower octane rating than is desired. Accordingly, such gasoline may sell for a lower price. In other cases, in order to maintain a high octane rating, a gasoline producer may have to add components (e.g., isooctane, aromatics, and the like) that are more expensive than olefins. In sum, the measurement errors associated with standard SFC and FIA have caused many gasoline producers to have lower profit margins than would have been possible if the producers had been able to more accurately measure the olefin levels in gasoline.
Although chemometric modeling may work well at accurately determining key gasoline parameters (e.g., olefin levels) for on-line and laboratory analysis of routine samples, chemometric modeling may not accurately determine gasoline properties for new types of gasoline blends or gasoline samples from different refineries that contain unique spectral features, which were not previously included in the model. Indeed, where the gasoline blend comprises a new spectral feature, the chemometric model may need to be updated in order to accurately predict desired parameters.
Thus, while techniques currently exist that are used to determine olefin levels in gasoline fuels, challenges still exist, including those listed above. Accordingly, it would be an improvement in the art to augment or even replace current techniques with other techniques.